1. Field of the Invention
The present invention relates to a camera system comprising an exchangeable lens having individual information of its own to be outputted to a camera body and the camera body which performs various operations using the information inputted thereto from the exchangeable lens. The present invention further relates to a camera system having a plurality of focus detection areas.
2. Description of the Prior Art
Individual information of lenses to be required to be controlled by a camera body increases as a camera system has more functions such as an automatic focusing control. Heretofore, information such as a fully open aperture value is mechanically transmitted from a lens to the camera body. Therefore, the number of kinds of information is small and the information cannot be transmitted from the lens to the camera body with a high accuracy. In order to overcome this problem, various camera systems are proposed. According to a known camera system, a read only memory (ROM) which stores the individual data of respective lenses is mounted in the lens so as to electrically transmit the lens data from the lens to the camera body.
The specification of German Patent No. 3518887 discloses a camera system in which a storing means provided in each of the lenses stores an electric signal peculiar to each of the lenses while the camera body stores the lens data on these lenses. According to this camera system, the lens data stored in the camera body is selected in response to an electric signal transmitted from the lens, whereby an exposure control appropriate to the lens can be made.
But as the camera system is provided with more and more functions such as a focus condition detection in a plurality of focus detection areas, i.e., when lens data to be controlled by the camera body increases further, the following problem occurs in the above-described cameras: if a conventional lens which includes the ROM which stores the lens data necessary only for the function of the conventional camera, is mounted on a developed camera body, the camera body can not operate the developed function operate thereof due to the shortage of lens data even though the mounting portion of the lens barrel can be attached to the mounting portion of the camera body.
The provision of a storing means in the camera body which stores the individual lens data of respective lenses does not allow the occurrence of such a problem, but the storing means is required to store a large amount of data therein, which is not preferable in view of practical use.
Next, lens data necessary for detecting a focus condition is considered. Heretofore, even if an object is sufficiently bright and has a high contrast, in some cases, a focus detecting operation must be controlled depending on the configuration and condition of a lens in a camera which has a single focus detection area and performs only the focus condition detection of an object present in an area including an optical axis. Such a control must be made in the following cases: The condition to be determined by diameter and position of the exit pupil of the lens such as a reflecting telephoto lens is not suitable for a focus detecting element; the light reflected from an object is not incident on an entire focus detecting element because the diameter of the exit pupil of a lens is too small (a vignetting occurs in this case); and a macro-photographing in which the position of the diameter of the exit pupil changes, the depth of focus becomes shallow, and the aperture value becomes great.
The specification of U.S. Pat. No. 4,509,842 discloses the following art. A signal, indicating that it is impossible to detect a focus condition, is stored in a storing means provided in a lens such as a reflecting telephoto lens. It is decided whether or not a fully open aperture value stored in the lens as lens data exceeds a predetermined value (for example, F=5.6). And a signal, indicating that a focus condition cannot be detected, is outputted from the lens when the lens is used to carry out a macro-photographing.
However, the above-described arts all relate to the camera system which has a single focus detection area and detects only the focus condition of an object present in an area including an optical axis, i.e., these arts cannot be applied to a camera system having a plurality of focus detection areas in a photographing picture plane. That is, the known camera system is provided with only data which indicates whether or not it is possible to detect a focus condition on an axial focus detection area. Contrary to such a camera system, in the camera system in which a plurality of focus detection areas are provided in the photo-taking picture plane, a focus detection area which can be used, is varied according to the condition determined by the configuration of the exit pupil of the lens and the diameter as well as the position of the exit pupil thereof. Therefore, it is necessary to store as lens data the information of respective lenses in the storing means provided in the lens, which indicates which of a plurality of focus detection areas can be used.
When the conventional lens used in the camera system having a single focus detection area is used in a camera system having a plurality of focus detection areas, as described above, the problem that the lens data is short occurs.
It is preferable that a focus condition is detected by using a great area in order to detect a focus condition with a high accuracy, but a vignetting occurs when a lens whose exit pupil is small (for example, a telephoto lens whose focal length is too long) is used although there is no problem with a lens whose exit pupil is great. In this case, a focus condition cannot be detected. In order to detect a focus condition not only for a lens whose exit pupil is great, but also for a lens whose exit pupil is small, it is necessary to prevent the occurrence of a vignetting. To this end, it is necessary to reduce a focus detection area, which means that a focus condition cannot be detected with a high accuracy.
That is, the known camera has a problem that the improved accuracy of a focus condition detection is inconsistent with the increase of the number of the kinds of lens which can be used for detecting a focus condition.
The above-described camera system has another problem which is described hereinbelow.
As described previously, in this kind of camera, various kinds of exchangeable lenses are used as a photo-taking lens. A photo-taking lens is composed of only a lens or of a lens and a converter lens for converting the focal length of the photo-taking lens.
The converter lens for use in this kind of camera transfers the lens data transmitted from the lens directly to the camera body in order to enable a focus detecting operation and converts the lens data, which is varied by the converter lens, such as the focal length or the fully open aperture value at the same rate of change as that of the focal length, thus outputting the converted data to the camera body.
Furthermore, according to the known camera body, whether or not a vignetting occurs is detected based on the fully open aperture value obtained from the lens. In this respect, it may be considered whether or not a focus condition can be detected by using the fully open aperture value converted by the converter lens when the lens is used with the converter lens mounted thereon as well.
But the position of the exit pupil of the lens attached to the converter lens may change along the optical axis to a greater extent than the position of the exit pupil of the lens not attached to the converter lens. In this case, it cannot be accurately decided whether or not a focus condition can be detected according to the information of the fully open aperture value which is converted at the same rate of change as that of the focal length due to the operation of the converter lens.
That is, in the known camera system, even though a vignetting does not occur due to the change of the position of the exit pupil, it is decided that the vignetting will occur only according to the information of the fully open aperture value, so that it can be decided that the focus condition can not be detected. In particular, in a camera system having a plurality of focus detection areas, a focus detection area which can be used for detecting a focus condition is changed greatly by the change of the position the exit pupil, thus causing the above problem.